Manufacture of optical elements

ABSTRACT

An optical element comprising a glass substrate portion on which a portion of a plastics material having approximately the same index of refraction as glass is molded. Shrinkage of the plastics material during curing is compensated for by allowing the glass substrate to deform due to the shrinkage, and optically working flat the outer surface of the glass substrate so that when the element is released from mold this outer surface will restore and take up a curvature which is equal to the amount of curvature lost at the molded optical surface due to the shrinkage.

0 United Stat 1111 3,917,766

Howden Nov. 4, 1975 1 MANUFACTURE OF OPTICAL ELEMENTS 3,171,869 3/1965Weinberg 264/1 3,211,811 10/1965 Lanman 264/1 [75] Inventor' w Howden,salfords near 3,497,577 2/1970 Wichterle 264/1 Redhlll, England3,542,907 11/1970 Wichterle 264/1 [73] Assignee: U.S. PhilipsCorporation, New

Y k N Y Primary ExaminerRobert F. White Assistant Examiner-Gene Auville1 Notice: The Pomon of thfi term of this Attorney, Agent, or Firm-FrankR. Trifari; George B.

patent subsequent to the Apr. 8, Berka 1992 has been disclaimed. I [22]Filed: Nov. 13, 1972 [57] ABSTRACT [21] AppL NO; 306 344 An opticalelement comprising a glass substrate portion on which a portion f aplastics material having approximately the same i dex of refraction asglass is [30] Fore'gn Apphcamm Pnomy Data molded. Shrinkage of theplastics material during cur- Nov. 25, 1971 United Kingdom 54807/71 ingis compensated for by allowing the glass substrate to deform due to theshrinkage, and optically working [52] US. Cl. 264/1; 264/162; 264/296flat the outer surface of the glass substrate so that [5 1] Int. Cl.B29D 11/00 when the element is released from mold this outer sur- [58]Field of Search 264/1, 2, 162.3, 296 face will restore and take up acurvature which is equal to the amount of curvature lost at the molded[56] References Cited optical surface due to the shrinkage.

UNITED STATES PATENTS 6 Claims, 4 Drawing Figures 2,332,930 10/l943Rinia 264/1 2,911,682 1l/l959 Ewald 264/1 US. Patent Nov. 4, 19753,917,766

MANUFACTURE OF OPTICAL ELEMENTS This invention relates to themanufacture of aspheric light-transmissive optical elements using areplication process in which at least that portion of the element whichdefines an optical surface thereof is moulded from a plastic material.

In the production of such optical elements there exists the problem offorming the optical surface to the desired accuracy due to the shrinkageof the pastics material when it is cast.

One solution to this problem is given by Starkie in an article on Pagesl07-109 of Modern Plastics, October 1947, in which there is describedthe manufacture of an aspheric optical element, such as a Schmittcorrector plate, by what is referred to as the surface finishingprocess. In this process a plastics material is cast in an opticallyworked mould which conforms to a desired shape of an optical surface.This produces a mouled preform which due to the shrinkage of theplastics material when it is cast has an optical surface which departsvery slightly from the exact shape of the interior of the mould. Usingthe same mould a thin film of the same plastics material as used for thepreform is then cast on the surface of the preform. This film isrelatively thin compared with the bulk of the preform, so that itsshrinkage on casting is small and the result is that an optical elementis obtained whose shape conforms fairly accurately to the shape of theinterior of the mould. Because the same material is used for the preformand the film, the interface between the two need not affect the opticalproperties of the element, the refractive index at each side of theinterface being the same. However, with this process the shrinkage ofthe film during moulding means that the optical element cannot conformexactly to the desired optical shape, but the tolerance involved isacceptable for some applications.

In another type of replication process, a moulded preform is dispensedwith and a surface coating of plastics material is moulded in accordancewith a desired aspheric shape on the surface of a substrate. In thisinstance, the surface coating is very thin compared with the thicknessof the substrate so that, as in the previous process, the shrinkage oncasting is small. One way to achieve greater accuracy with this lattertype of process is to compensate for shrinkage of the plastics materialduring casting by using a mould having an enlarged or exaggeratedaspheric surface such that the quantity of plastics material needed inthe mould to fill it initially will shrink on casting to form therequired aspheric surface. This way of achieving greater accuracy isessentially on a trial and error basis and involves comparingsuccessively moulded elements with an optically worked master, the mouldbeing fractionally enlarged after each comparison until the desiredaccuracy is achieved. Another way to achieve greater accuracy with thislatter type of process involves compensating for shrinkage of theplastics material during casting by using a material whose refractiveindex is greater by an appropriate amount, than that of a glass masterin respect of which the aspheric surface of the mould is complementary,to offset the reduced optical thickness caused by the shrinkage of theplastics material.

The present invention proposes a further way of achieving greateraccuracy with said latter type of process.

According to the present invention, a method of manufacturing anaspheric light-transmissive optical element is characterized insupporting a light-transmissive substrate with one surface in proximitywith a mould surface that has a profile which is the negative of arequired optical profile, filling the region between said surfaces witha light-transmissive adhesive plastics material having, when cast,approximately the same refractive index as the substrate, causing theplastics material to cast into a moulded layer with it adhered to bothsaid surfaces and with the substrate in a state of elastic deformationdue to shrinkage of the plastics material on casting, optically workingflat the other, outer, surface of the substrate, and releasing thesubstrate with the mould layer adhered thereto from the mould surface,said substrate and layer together forming said optical element.

With the method of manufacturing according to the invention, the step ofoptically working flat the outer surface of the substrate results inthis outer surface assuming, when the substrate is restored from itselastic deformation when it is released with the moulded layer from themould surface, a profile which represents a curvature equal to theamount of curvature lost at the aspheric surface of the moulded layerdue to shrinkage of the plastics material on casting. It will beappreciated that this loss of curvature only becomes apparent when thesubstrate is restored from its elastic deformation by separating theaspheric surface of the moulded layer from the mould surface.

In carrying out the invention the use of a thermosetting syntheticmaterial, such as an epoxy resin or a polyester resin, for the mouldedlayer is preferred because such materials are relatively hard when castand thus give a durable product. The substrate may also be a mouldedpreform of such materials, but the use of a glass substrate is preferredbecause glass blanks with flat surfaces are easily and cheaplyobtainable and need not be manufactured specially for the purpose.Ideally, the refractive index of the plastics material is the same asthat of the substrate, so that the curvature on said outer surface ofthe substrate compensates exactly for the loss of curvature at theaspheric surface of the moulded layer. However, in practice,approximately the same refractive indices will give a high order ofaccuracy. For example, for a glass substrate having a refractive indexof, say, 1.62, the plastics material may be an epoxy resin having arefractive index of about 1.57. This will give an error or deviation ofonly 1 micron for a moulded layer of l mm thickness.

Also, in carrying out the invention, the region between the mouldsurface and the facing surface of the substrate is preferably bounded bya member on which said substrate is supported and which is displaceablewith respect to the mould surface in a direction normal to the substrateto allow deformation of the latter adjacent its edge.

In order that the invention may be more fully understood reference willnow be made by way of example, to the accompanying drawing of which:

FIG. 1 shows diagrammatically and not to scale an optical elementmanufactured in accordance with the invention; and

FIGS. 2 to 4 illustrate respective stages in the manufacture of theoptical element of FIG. 1.

Referring to the drawing, the optical element shown in FIG. 1 is aSchmidt corrector plate and comprises a circular glass substrate 1having one surface 2 coated with a moulded layer 3 of an epoxy resin.The outer surface 4 of the layer 3 is aspherical and conformsapproximately to the well-known Schmidt corrector plate profile. Theouter surface 5 of the glass substrate ll has a curvature equal to theamount of curvature which is deficient at the layer surface 4 due toshrinkage of the epoxy resin on casting this layer using a mould surfacewhich is complementary to the full Schmidt corrector plate profile. Theglass substrate l typically has a diameter of 12 cm, a thickness of 0.6cm and a refractive index of 1.62. Suitably, the layer 3 is moulded frombisphenol A epoxy resin having an epoxy equivalent of about 5 and anhydroxyalkylated polyamine hardenenr which produces a ct: red resinhaving a refractive index of 1.57. The difference in thickness betweenthe thickest and thinnest parts of the layer 3 is typically less than0.02 inches.

The manufacture, in accordance with the invention, of the correctorplate of FIG. 1 will now be considered with reference to FIGS. 2 to 4. Aglass mould 6 has a mould surfaace 7 that has a profile which is thenega tive (i.e., the complement) of the required optical profile for theCorrector plate. The mould surface '7 is given the required profile bynormal optical working techniques and this profile is complementary tothe aspheric profile of a glass master of known refractive index (i.e.,1.57 the same as the resin layer 3) whose optical properties thecorrector plate to be manufactured has to conform to. The accuracy ofthe mould surface profile is checked by comparison with the glass masterby interference fringe methods.

The mould surface 7 is then processed to enable the epoxy resin mixturewhich is to form the layer 3 to be released from it without unduedifficulty after the resin has cured. To this end the surface 7 iswashed in a mix ture of warm water and detergent, cleaned withprecipitated chalk, washed with deionised water, rinsed in isopropropylalcohol and then cleaned by ion bombardment in a vacuum chamber, andfinally coated with silicone release agent and polished. The surface 2of the glass substrate 1 is similarly treated to enable the epoxy resinmixture to adhere to it as well, but is not coated with release agent.

The glass substrate 1 is then placed in close proximity to the glassmould 6 in the configuration shown in FIG. 2, the spacing between thesurface 2 and 7 being typically 0.05 to 0.12 cm at the narrowest points.Accuracy of this spacing is facilitated by means of a sleeve 8 on whichthe glass substrate 1 is supported and which is free to slide, under theinfluence of an applied force, with respect to the glass mould 6. Thespace 9 thus formed between the surfaces 2 and '7 is then filled withthe mixture of epoxy resin and hardener. It is important that airbubbles are not trapped in the resin mixture while this is being doneand, to prevent this, this step is carried out in vacuum chamber. Theresin mixture is then left to cure at room temperture to form themoulded layer 3. Since the resin mixture shrinks on (g, curing, and alsoadheres to both the surfaces 2 and 7 sufficiently well to prevent thebond being broken under the pressure exerted in a direction normal tothe glass substrate 1 due to the shrinkage, the glass substrate llassumes a state of elastic deformation as illustrated in FIG. 3. Theupper surface 9 of the glass substrate 1 now has a profile which is indirect proportion to the shrinkage factor of the resin mixture and thepro file of the mould surface 7. This upper surface 9 is now ground andpolished flat to the surface 10 (S-FTG. 1) as illustrated in FIG. 4,after which the glass substrate l with the layer 3 adhered to itssurface 2 is released from the mould surface 7 by the discreteapplication of a force specifically higher than the force of adhesionbetween the cured resin mixture and the mould surface 7. On release, theglass substrate ll restores from its elastic deformation, therebycausing the aspheric surface 4 to lose a small amount of curvature as itis straightened. However, this loss of curvature is compensated for bythe curvature which the worked (previously flat) surface 10 (5) assumeswhen the glass substrate 11 restores from its elastic deformation.

What we claim is:

l. A method of manufacturing an aspheric lighttransmissive opticalelement, comprising the steps of supporting a light-transmissiveelastically deformable flat substrate with one surface above a moldsurface that has a profile which is the negative of a required op ticalprofile, filling the space between said surfaces with alight-transmissive adhesive shrinkable plastic material havingapproximately the same refractive index as the substrate, curing theplastic material to form a molded layer adhered to both said surfacesand, due to shrinkage, to elastically deform said substrate, opticallyworking flat the other, outer, surface of the deformed substrate, andreleasing the substrate with the molded layer adhered thereto from themold surface whereby said substrate resumes at least partially itsoriginal undeformed condition thus forming together with said moldedlayer said optical element.

2. A method as claimed in claim 1, for which said plastics material is athermosetting synthetic material.

3. A method as claimed in claim 2, wherein said thermosetting syntheticmaterial is an epoxy resin or a polyester resin.

4. A method as claimed in claim 2, wherein said substrate is of glass.

55. A method as claimed in claim 1, for which the space between themould surface and the facing surface of the substrate is bounded by amember on which said substrate is supported and which is displaceablewith respect to the mould surface in a direction normal to thesubstrate.

6. A method as claimed in claim 1 wherein a release agent is applied onthe mold surface prior to the filling of said plastic material.

1. A METHOD OF MANUFACTURING AN ASPHERIC LIGHT-TRANSMISSIVE OPTICALELEMENT COMPRISING THE STEPS OF SUPPORTING A LIGHT-TRANSMISSIVEELASTICALLY DEFORMABLE FLAT SUBSTRATE WITH ONE SURFACE ABOVE A MOLDSURFACE THAT HAS A PROFILE WHICH IS THE NEGATIVE TO A REQUIRED OPTICALPROFILE FILLING THE SPACE BETWEEN SAID SURFACES WITH ALIGHT-TRANSMISSIVE ADHESIVE SHRINKABLE PLASTIC MATERIAL HAVINGAPPROXIMATELY THE SAME REFRACTIVE INDEX AS THE SUBSTRATE CURING THEPLASTIC MATERIAL TO FORM A MOLDED LAYER ADHERED TO BOTH SAID SURFACESAND DUE TO SHRINKAGE TO ELASTICALLY DEFORM SAID SUBSTRATE OPTICALLYWORKING FLAT THE OTHER OUTER SURFACE OF THE DEFORMED SUBSTRATE, ANDRELEASING THE SUBSTRATE WITH THE MOLDED LAYER ADHERED THERETO FROM THEMOLD SURFACE WHEREBY SAID SUBSTRATE RESUMES AT LEAST PARTIALLY ITSORIGINAL UNDEFORMED CONDITION THUS FORMING TOGETHER WITH SAID MOLDEDLAYER SAID OPTICAL ELEMENT.
 2. A method as claimed in claim 1, for whichsaid plastics material is a thermosetting synthetic material.
 3. Amethod as claimed in claim 2, wherein said thermosetting syntheticmaterial is an epoxy resin or a polyester resin.
 4. A method as claimedin claim 2, wherein said substrate is of glass.
 5. A method as claimedin claim 1, for which the space between the mould surface and the facingsurface of the substrate is bounded by a member on which said substrateis supported and which is displaceable with respect to the mould surfacein a direction normal to the substrate.
 6. A method as claimed in claim1 wherein a release agent is applied on the mold surface prior to thefilling of said plastic material.